In the known methods a glass panel is press-formed which usually takes place at very high temperatures (1000° C.-1100° C.). In this manner a glass face panel can be formed. Cathode ray tubes, for example, comprise such a glass panel.
Cathode ray tubes (CRT's) are becoming of ever more greater size, thus increasing the weight of the CRT's. Furthermore the front surface of the glass panel is becoming ever more flatter. However, increasing the flatness of the front surface of the face panel generally increases also the weight of the glass panel because the thickness of the glass panel has to be increased to ensure safety against implosion or explosion of the CRT.
There exists a great need for increase in strength of the CRT, and in particular of the glass panel, without increasing the weight. An increase in the strength of the glass panel may improve the yield.
The present invention is aimed at providing a method which enables increasing the yield of the method and/or reducing the weight of the glass panel.
To this end the method in accordance with the invention is characterized in that during at least a part of the second stage the heat radiation of the outer face portion of the panel is reduced to reduce a difference in cooling rate between the inner and outer face portion.
The invention is based on the insight that during the cooling stage inhomogeneities in the stress level in the panel can occur. In order to improve the strength of (Real Flat) display panels the cooling velocity after press forming is usually increased to a such extent that a compressive stress is produced in the surface in the order of 3 to 25 Mpa. Panels of this type are called High Surface Compression panels. Trying to correct for the above inhomogeneities in the stress level during a final annealing step would easily affect the necessary compressive stress (surface stress). This reduces the yield and can seriously affect the safety of the tubes. This is in particular important for panels with an (almost) flat inner and/or outer surface such as Real Flat panels.
The intention of the inventive cooling process is to render the cooling rates of the inner and outer face portion more symmetrical. By this is meant that during cooling the inner and outer face portion pass at substantially the same moment the temperature Tg (which is the temperature at which the glass changes from the viscous to the solid state). A reduction in cooling can be advantageously realized by placing a heat reflection means opposite to the outer face portion of the panel.
Reducing the heat radiation of the outer face portion of the panel has been found to be an effective means to reduce temperature gradients over the panel thickness. Radiation transport is responsible for 40-70% of the heat transport. Cooling with air of other portions alone is less efficient. This effect may for instance be used in practice to manufacture panels with a lower weight, or panels with a flatter front surface, or to reduce the fall-out, i.e., the percentage of panels that does not pass safety tests, or any combination of these beneficial effects.
According to a first embodiment a heat reflection means is arranged at a position facing the outer face portion of the panel, advantageously a heat reflection means is used in combination with a heating means for a stronger effect.
According to a further embodiment a heat reflection means is used which comprises at least one plate made of a material selected from the group comprising Ni, Al, Au, Al-oxide, or covered with a such material.
To increase the effect of the heat reflection means a heat absorption means may be arranged facing the inner face portion of the glass panel.
In view of the above it is in particular an aspect of the invention to provide a display panel of the High Surface Compression type which presents substantially no tensile stresses in the (view window) area between the inner and outer face portion of the panel.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.